1. Field of the Invention
The present invention relates generally to acetabular cups. More specifically, it relates to an acetabular cup having optionally removable portions to permit a choice between methods of affixation.
2. Description of the Invention Background
Acetabular cups are used to replace the natural acetabulum, or hip socket, in instances of degenerative bone disease or severe fractures. Acetabular cups typically comprise a metallic cup which is fitted into a portion of the acetabulum which has been reamed to provide a mating fit with the cup. The inside of the metallic cup is fitted with a cup-shaped liner formed of ceramic or plastic, such as polyethylene. The femoral head (natural or prosthetic) fits into the cup-shaped liner. The metallic outer cup is presently held in place in the reamed natural acetabulum either by friction fit with or without cement, or by one or more screws.
Typically, a surgeon does not decide which method of affixation to use until the operation is underway. Only at this point can the surgeon directly observe the condition of the acetabulum to determine which method of affixation is desirable. In addition, if affixation by means of bone screws is adopted, one cannot determine in advance where the best place for insertion of a bone screw will be. Acetabular cups are, therefore, provided with a plurality of holes over the surface thereof for receiving bone screws if and where required. Metallic cups which are to be held in place by friction fit and cement, however, do not require holes. Even when screws are used to secure the cup to the natural bone, all of the holes provided in the metallic cups need not be used.
Experience has shown, however, that it is undesirable to implant a metallic cup with unused screw holes. The unused holes have been blamed for problems following surgery. Holes remaining in the wall of the cup allow debris to pass through and retard bone growth. The edges of the holes rub against the plastic liners to create plastic shavings. These shavings deteriorate the bone of the acetabulum. Also the shavings irritate the area where the bone is absorbed into the cup.
Zarnowski, U.S. Pat. No. 4,955,325, discloses an acetabular cup system capable of conversion from a cup constructed for affixation by fasteners to a cup suitable for cemented affixation. Zarnowski comprises a cup with a plurality of holes therein, spacers, and a spacer inserter for selectively inserting the spacers into the holes. The cup is designed with holes suitable for affixation with bone screws. The surgeon then has the option to decide after the operation is underway to use a cemented method of affixation. Using the spacer inserter tool, the user may snap spacers into the holes. The spacers project from the outer surface of the cup to maintain a gap for a layer of cement.
However, in Zarnowski, the spacers themselves have holes therein. Furthermore, the system disclosed in Zarnowski leaves extra holes in the cup when fewer screws are used to secure the cup to the natural bone than the number of holes provided in the cup. Additionally, the spacers are separate components, constituting multiple parts which must be stored.
There is a need for an acetabular cup which provides for the options between different methods of affixation and which eliminates unnecessary holes in the acetabular cup. There is a further need for an acetabular cup which does not include multiple components.